This type of support may in particular be a barge of the floating production storage and offloading (FPSO) or floating storage regasification unit (FSRU) type for processing and storing LNG, or it may be a ship, in particular having a hull and storage tanks made of steel or of concrete as described in WO 01/30648, and as explained below.
The methane-based natural gas is either a by-product of oil fields, being produced in small or medium quantities, in general in association with crude oil, or else it is a major product from a gas field, where it is to be found in combination with other gases, mainly C-2 to C-4 alkanes, CO2, nitrogen, and traces of other gases. More generally, the natural gas comprises a majority of methane, preferably at least 85% methane, with the other main constituents being selected from nitrogen and C-2 to C-4 alkanes, i.e. ethane, propane, and butane.
When the natural gas is associated in small quantities with crude oil, it is generally processed and separated and then used on site as fuel in boilers, gas turbines, or piston engines in order to produce electricity and heat for use in the separation or production processes.
When the quantities of natural gas are large, or indeed substantial, they need to be transported so that they can be used in regions far away, in general on other continents, and in order to do this the preferred method is to transport the gas while it is in the cryogenic liquid state (−165° C.) and substantially at ambient atmospheric pressure. Specialized transport ships known as “methane tankers” possess containers of very large dimensions and they present extreme insulation in order to limit evaporation while traveling.
On oil fields in open sea, situated a long way off-shore, petroleum fluids such as crude oil or gas are generally recovered, processed, and stored on board a said floating support 10 often referred to as an FPSO. Petroleum fluids such as crude oil and/or gas are then transferred to offloading ships 20 that call regularly, e.g. every week, to recover the production from the oil field and to export it to places where it is consumed. When transferring liquefied gas of the LNG type at −165° C., transfer devices include at least one pipe 17 for removing gas from the tanks of the offloading ship 20 progressively as they are filled with LNG, and in particular for removing methane gas so that it can be reliquefied on board the FPSO 10, as described below with reference to FIG. 9A.
Another technical field is one in which LNG is stored at sea close to a utilization site, e.g. in order to deliver gas to land after it has been regasified, or else to transform it on site, on-board the floating support, into electricity and for delivering said electricity to the local network on land. Under such circumstances, the ship comes to unload its cargo of LNG and the floating support is referred to as an FSRU.
The term “processor installation” is used more particularly below to designate any installation for liquefying natural gas into LNG, any installation for regasifying LNG, and/or any installation for transferring LNG between said support and a methane tanker type ship for offloading and storing LNG, which tanker may be positioned in tandem or else drawn up alongside said support.
Processor installations of this type have means or components such as pumps, flow pipes, compressors, heat exchangers, expander devices, generally in the form of decompression turbines, cryogenic heat exchangers, and containers, and also connection pipes and connection elements between these various devices.
Leaks of the liquid being processed and stored, in particular of LNG liquefied gas where appropriate, may arise either from valves, pumps, heat exchangers, containers, or pipes, or else more particularly from gaskets in coupling elements or in said components, or indeed from breakage of one or more of those various components.
Leaks of LNG are particularly dangerous for three reasons:
1) Spilt LNG gasifies quickly on making contact with air and solid surfaces, and by mixing with ambient air it creates a very dangerous mixture that explodes in the presence of the slightest spark or slightest hot point.
2) Equipment conveying or containing LNG (−165° C.) is made out of material capable of withstanding such cryogenic temperatures, in general nickel-based steels or indeed Invar. Such special steels are very expensive, and in general they are not used for support elements or for the structure of the FPSO, which are generally made out of ordinary steels. However, such ordinary steels become brittle on contact with very low temperatures and they lose their mechanical strength, thus running the risk of structural elements breaking, and possibly even of the deck of the FPSO breaking in the event of a major leak directly onto said deck, unless critical locations are protected by insulating materials that are themselves very good at withstanding cryogenic temperatures.
3) Sudden contact between large quantities of LNG and seawater is very dangerous, since the LNG (−165° C.) is heated suddenly by seawater at a temperature in the range 10° C. to 20° C., and it is heated in the presence of air, and thus in the presence of oxygen, thereby creating major risks of immediate explosion.
Means for connecting LNG leaks are generally installed in register with critical pieces of equipment so as to direct said LNG towards containers where the LNG is recuperated and then either regasified and delivered to a flare in order to be eliminated, or else transferred by pumping to one of the storage tanks of the FPSO. Such collector means are generally uncovered, and as a result they may also collect all kinds of pollution, such as oils leaking from various machines, e.g. rotary machines such as compressors, or indeed run-off water and rain or spray, generally carrying dust. Such water and oil is generally directed towards settling tanks in which, in conventional manner, the oil, the water, and the solid particles, if any, separate naturally merely under gravity, because of their different densities.
The fact that the cryogenic liquid, and in particular LNG at −165° C., is present in the polluting fluids makes direct use of such settling tanks impossible and it is necessary beforehand to separate the LNG from the other polluting agents before they enter into said settling tank.